By Joshua Bickel and Julián Trejo Bax
Published on
Billions fewer birds are flying through North American skies than decades ago and their population is shrinking ever faster, a new study has found. The decline is mostly due to a combination of intensive agriculture and warming temperatures.
Nearly half of the 261 species studied showed big enough losses in numbers to be statistically significant and more than half of those declining are seeing their losses accelerate since 1987, according to the paper published in journal Science.
The study is the first to look at more than the total bird population by examining the trends in their decrease, where they are shrinking the most and what the declines are connected to.
“Not only are we losing birds, we are losing them faster and faster from year to year,” says study co-author Marta Jarzyna, an ecologist at Ohio State University.
Are shrinking bird species at risk of extinction?
The only consolation is that the birds that are shrinking in numbers the fastest are species – such as the European starling, American crow, grackle and house sparrow – with large enough populations that they aren’t yet at risk of going extinct, says study lead author Francois Leroy, also an Ohio State ecologist.
Cornell University conservation scientist Kenneth Rosenberg, who wasn’t part of the study, says the species declining fastest in the new research “are often considered pests or ‘trash birds,’ but if our environment cannot support healthy populations of these extreme generalists and extremely adaptable species that are tolerant of humans, then that is a very strong indicator that the environment is also toxic to humans and all other life.”
The same bird species showed the biggest population drops in Rosenberg’s 2019 study that found North America had 3 billion fewer birds than in 1970, but didn’t look at changes in the rate of loss or causes.
Where are birds declining fastest?
The biggest locations for acceleration of bird loss were in the Mid-Atlantic, the Midwest and California, the study found. And geography proved important when Leroy and Jarzyna looked for reasons why so many bird species are shrinking ever faster.
When it came to population declines – not the acceleration – the scientists noticed bigger losses further south, such as in Florida and Texas. When they did a deeper analysis they statistically connected those losses to warmer temperatures from human-caused climate change.
“In regions where temperatures increase the most, we are seeing strongest declines in populations,” Jarzyna says. “On the other hand, the acceleration of those declines, that’s mostly driven by agricultural practices.”
The scientists found statistical correlations between speeded-up decline rates and high fertiliser use, high pesticide use and amount of cropland, Leroy says. He adds that they couldn’t say any of those caused the acceleration of losses, but it indicates agriculture in general is a factor.
Why are birds important?
More than just wildlife, birds are fundamental to our ecosystems.
“We see birds being good pollinators. We see them dispersing seeds, we see them as pest control, they play really important functions,” says Jarzyna. “And if they disappear, those functions are gone as well. So from the perspective of ecosystem functioning, it’s really important that our birds don’t go away.”
But habitat destruction is widespread.
“Basically every single habitat type, so species that breed in things like grasslands or deserts or marshes and so on, is declining with exception of forests, of forest birds,” says Jarzyna. “So we need to ask ourselves a question. How do we protect these groups of birds?”
Video editor • Denis Loctier